The invention relates generally to the field of brake systems. More precisely, the invention relates to the operation of a motor-vehicle brake system in a braking situation in which differing coefficients of friction of the road prevail on opposite sides of the vehicle (μ-split situation), or in comparable braking situations.
It is generally known that when the brakes of a motor vehicle are applied in a μ-split braking situation the vehicle has a tendency to rotate about the vertical axis of the vehicle (also called yawing). FIG. 1 illustrates this yawing in connection with a motor vehicle 10, the left wheels 12, 14 of which are running on ice, and the right wheels 16, 18 of which are running on dry asphalt.
The coefficient of static friction of ice amounts to approximately μs=0.1, whereas the corresponding value of dry asphalt is approximately μs=0.8. By reason of these greatly differing coefficients of friction, the wheels 12, 14 on the low-friction-coefficient side (on the left in FIG. 1) attain a state that requires a slip regulation by an anti-lock system (ABS) more quickly than the wheels 16, 18 on the high-friction-coefficient side (on the right in FIG. 1). By reason of this asymmetrical slip regulation, when the brakes of the motor vehicle 10 are applied differing braking forces act on the left wheels 12, 14 and on the right wheels 16, 18, the difference of which at the front wheels 12, 16 may be particularly strongly pronounced by reason of the dynamic axle-load shift. These differing braking forces lead, in turn, to a torque about the vertical axis 20 of the vehicle (the so-called yawing moment), and hence, under certain circumstances, to a yawing of the motor vehicle 10.
In the case of heavy motor vehicles the yawing illustrated in FIG. 1 occurs so slowly that, with slip regulation activated, it can be compensated by a driver sufficiently quickly by steering in the opposite direction. Above all in the case of lighter motor vehicles, however, measures have to be taken additionally in order to assist the driver in the course of braking in a μ-split situation.
One possibility to counteract the build-up of a yawing moment in a μ-split situation is the implementation of a so-called select-low regulation in the ABS control software. With such a regulation, in the case of a detected μ-split situation the braking force at the wheels of the rear axle is set in accordance with the ABS-regulated braking force on the low-friction-coefficient side. Whereas in the case of the select-low regulation a yawing can be very largely avoided and the controllability of the vehicle is therefore preserved well, a considerable underbraking of the wheels on the high-friction-coefficient side arises. This underbraking results in an unacceptable lengthening of the braking distance.
For this reason, in L. M. Ho et al., The Electronic Wedge Brake—EWB, XXVIth International μ Symposium 2006, pages 248f, a description is given of allowing a small difference in braking force at the opposite wheels of each axle (that is to say, between the wheels on the high-friction-coefficient side and the wheels on the low-friction-coefficient side). The braking-force difference is then gradually increased, in axle-specific manner, up to a defined value. The gradual axle-specific increase of the braking-force difference results only in a slow build-up of yawing moment. In each case the build-up of yawing moment is distinctly delayed in comparison with a ‘pure’ ABS regulation. The driver therefore has sufficient time to compensate a possibly resulting yawing of the vehicle by means of steering movements.
In FIG. 2 the ramp-like increase of the braking-force differences in combination with a select-low regulation is illustrated in a braking-force/time diagram according to L. M. Ho et al. In FIG. 2 it is assumed that (as represented in FIG. 1) the left side of the vehicle is the low-friction-coefficient side and the right side of the vehicle is the high-friction-coefficient side. Accordingly, at the left front and rear wheels (FL/RL in FIG. 2 and reference symbols 12 and 14 in FIG. 1) only low braking forces can be generated, whereas at the right front and rear wheels (FR/RR in FIG. 2 and reference symbols 16 and 18 in FIG. 1) distinctly higher braking forces can be built up. Overall, the braking distance in this case can be distinctly reduced in comparison with a ‘pure’ select-low regulation. Simultaneously, the driver is still given enough time to react to a possibly incipient yawing by steering in the opposite direction.
Now, it has turned out that in the case of the adapted select-low regulation illustrated in FIG. 2 an underbraking of the wheels on the high-friction-coefficient side still occurs. In other words, in μ-split situations the braking distance is frequently still unnecessarily long. With a view to shortening the braking distance, it is proposed in DE 10 2008 027 093 A1 to carry out a braking-force regulation in a μ-split situation with the proviso of the setting of a side-slip angle different from zero. This means that a slight yawing of the vehicle is permitted selectively. The braking-force regulation at the individual wheels is effected in this connection on the basis of a desired side-slip angle to be set within the range between 0.5° and 8°.
Conventional regulating strategies of the ABS control software in μ-split situations uncouple the driver (i.e. the brake pedal) from the braking-force regulation. For this purpose, ABS slip-regulating valves, which have been arranged between a hydraulic-pressure generator (for example, the master cylinder), on the one hand, and the wheel brakes covered by the braking force regulation, on the other hand, are closed. For the driver, the closing of the slip-regulating valves becomes noticeable in a ‘hard’ pedal feedback, since no more hydraulic fluid can be displaced out of the master cylinder to the wheel brakes.
In order nevertheless in the case of closed slip-regulating valves to be able to register a wish of the driver with respect to an increase in braking force, a pressure sensor for registering the master-cylinder pressure may be provided. An increase of the master-cylinder pressure registered by this pressure sensor with slip-regulating valves closed indicates a further depression of the brake pedal by the driver and can be taken into consideration in suitable manner within the scope of the ABS regulating operation.